FIG. 1 is a schematic circuit diagram of a conventional bandpass filter 100. The conventional bandpass filter 100 comprises a capacitor C1, a capacitor C2, a capacitor C3, an inductor L1 and an inductor L2. One terminal of the capacitor C1, the capacitor C3 and the inductor L1 are electrically connected to a first I/O (input/output) terminal TA of the filter 100. One terminal of the capacitor C2, the other terminal of the capacitor C3 and one terminal of the inductor L2 are electrically connected to a I/O terminal TB of the filter 100, wherein if the I/O terminal TA is an input terminal, the I/O terminal TB is an output terminal, or vice versa. The other terminal of the capacitor C1, C2, L1 and L2 are connected to a ground voltage GND. The conventional bandpass filter 100 is fabricated by LTCC (Low Temperature Co-Fired Ceramics).
FIG. 2 is a frequency-response diagram of the circuit shown in FIG. 1. The filter 100 has a resonant frequency f0 in the center of the passband, and there is a notch on the left-side band of f0 (which means the range smaller than f0) at the position about 1.9 GHz. The notch means that the filter 100 will cause larger attenuation at the frequency herein. It can be seen clearly from FIG. 2 that the attenuation on the right-side band of f0 (the range larger than f0) is not as ideal as the attenuation on the left-side band of f0, but this frequency response is acceptable in some application conditions. However, due to some limitation of regulations, application environments or specification of products, the attenuation on the right-side band of the resonant frequency f0 of the conventional bandpass filter 100 might not meet the requirement of them. For example, some regulations or specification of products require that the attenuation near a certain frequency (such as two times the resonant frequency, i.e. 2 f0) on the right-side band of the resonant frequency f0 should achieve a rated quantity (such as −35 dB), and it is thus very limited for the conventional bandpass filter 100 to apply.